The present invention is directed toward the field of power input protection circuits for power supplies. In particular, a power input protection circuit is disclosed for use with remotely powered electronic devices. The present invention protects remotely powered electronic devices from hazards such as lightning strikes and power cross conditions by limiting the amount of voltage and current the remotely powered electronic device can be subjected to.
In remotely powered electronic devices, the high-voltage power lines from the local utility company are often the primary source of input power for the device. The remotely powered electronic devices must be able to accept the power supplied by the high-voltage power lines and convert it to lower d.c. voltage levels that are compatible with the electronic components within the remotely powered electronic device.
An exemplary remotely powered electronic device is an Optical Network Unit ("ONU"). An ONU is a device that is used as an interface between fiber optic telecommunication lines and traditional wires used to provide telecommunication services such as cable television and telephonic services to homes or other buildings.
The power supply in an ONU receives high-voltage d.c. power from the local telephone company and converts it to lower level d.c. voltages for use by other circuitry within the ONU. The power supply in an ONU typically includes: (i) input protection and filter circuitry; (ii) energy storage circuitry, (iii) input voltage monitors and threshold circuitry, (iv) d.c. to d.c. converters; (v) ringing generators, and (vi) alarm and digital interface circuitry.
The ONU, a remotely powered electronic device, is provided with power through a long pair of telephone wires of up to 6000 feet long The long power input wires are susceptible to hazards such as high-voltage lightning strikes and power cross conditions resulting from power lines falling across power input wires. As a result, the ONU could be subjected to around 600 volt R.M.S. power surges which, if adequate input protection circuitry is not provided, could damage circuitry within the ONU.
Primary protection gas tubes are used to limit the amplitude of any high-voltage surge to .+-.1000 volts before the connection to the power supply. The input protection and filter circuitry, therefore, must be capable of handling high-voltage surges of up to 1000 volts to protect both the circuitry within the power supply unit and the other circuitry within the ONU. The present invention provides an input protection circuit that allows the circuitry within the ONU to survive after a lightning strike or power cross condition and to prevent the ONU from being destroyed if such a condition occurs. Also, many of the components within the ONU could be damaged by excessive current flow. Therefore, the present invention provides an input protection circuit that limits the flow of current.
Presently known methods for providing power input protection circuits include the circuit disclosed in FIG. 1. In this figure, input power is provided across input lines 1 and 2. A diode bridge network 3 receives the input power, rectifies it, and outputs the rectified input voltage at nodes 4 and 5. Series resistors 6 and 7 are provided between rectifier nodes 4 and 5 and the clamped output nodes 9 and 10, which serve as the input power connection points for the remainder of the power supply. A zener diode 8, such as an 150 volt zener diode, is bridged across clamped output nodes 9 and 10. When the input lines are subjected to voltages of up to 1000 volts, the zener diode 8 will allow the voltage supplied at clamped output nodes 9 and 10 to exceed 150 volts. The series resistors 6 and 7 will sustain the extra voltage applied across input lines 1 and 2 and pass the current.
This technique, shown in FIG. 1 is disadvantaged, first of all, because it requires sizeable series resistors 6 and 7 to sustain the extra voltage. Proper ventilation and heat sinking will be needed to dissipate the heat generated in resistors 6 and 7. If this circuitry were used in the ONU, for example, a 5.2 .OMEGA., 10 to 20 W resistor would be required per line. Another disadvantage of this method is inefficiency, Because the ONU draws approximately 0.5 Amps of current during normal operation, a lot of wasted power will be dissipated in the series resistors 6 and 7 during normal operation of the ONU Assuming the 5.2 .OMEGA. resistors noted above are used in the ONU, approximately 2.6 Watts of power would be dissipated in series resistors 6 and 7. Yet another disadvantage is the inefficiency caused by the power loss in the diode bridge network 3. The diode bridge 3 inserts two diodes in series during normal operation of the ONU. Each diode drops 1 volt resulting in an additional 1 W of wasted power that could otherwise be used for telecommunication services in the ONU.
Therefore, there remains a need in this art for a more efficient power input protection circuit. There remains a more particular need for an efficient power input protection circuit that does not require sizeable resistors to sustain excess voltage and the attendant ventilation and heat sinking requirements. In addition, there remains a more particular need for an efficient power input protection circuit that also provides a mechanism for limiting the flow of current through the power supply.